In electromagnetic geophysical exploration, electromagnetic signals are received and processed to provide an indication of the underlying geology. The electromagnetic signals can be either naturally produced, as in the case of magnetotelluric exploration, or artificially produced, as in the case of controlled-source electromagnetic exploration. The electromagnetic signals are received by one or more receiving sensors, which are typically placed on the ground. The receiving sensor is typically either a magnetometer, which measures the magnetic field, or an induction coil, which measures the time rate of change of the magnetic field.
The receiving sensor and the resulting data produced by the sensor in an electromagnetic geophysical survey system are susceptible to impulsive noise from a variety of sources. One common type of impulsive noise is referred to as motion noise. Motion noise is caused by movement of the receiving sensor in the earth's magnetic field The receiving sensor can be moved by wind gusts, microseismic activity in the ground, nearby vehicular traffic (particularly if the receiving sensor is located near a road) and nearby foot traffic. In the prior art, attempts have been made to protect the receiving sensor from motion noise by placing it in a lined pit beneath the surface of the ground. Such a procedure is not only too time consuming and expensive, but ineffective as well, because the receiving sensor still exhibits motion noise. Other types of impulsive noise include sferics, which are caused by distant lightning, and perturbations of the earth's magnetic field caused by nearby vehicular traffic.
One type of prior art technique to reduce impulsive noise involves stacking the received electromagnetic data. In stacking, multiple received signals are averaged together; continuous non-impulsive noise is reduced to acceptable levels by averaging a certain number of signals. In order for the randomly occurring impulsive noise to be reduced sufficiently, a large number of received signals is typically needed. Practically, it is too time consuming and expensive to acquire enough data to effectively reduce impulsive noise to acceptable levels by stacking.
Another type of prior art technique is disclosed is Buselli et al. U.S. Pat. No. 4,247,821. A sferics detector is connected to the same receiving antenna that acquires the electromagnetic signal. When sferics noise causes the received electromagnetic signal to exceed a threshold, the sferics detector causes an analog-to-digital converter, that converts the received signal, to freeze. The problem with this technique is that the noise is monitored together with the electromagnetic signal of interest, and it is difficult to distinguish between the two signals. This makes it difficult to remove the noise. Furthermore, the impulsive noise introduces a bias into the data, which bias cannot be discerned.